How much of the uterus does the fetus fill by 38-40 weeks?

At 38 to 40 weeks of gestation, the fetus has typically reached a size where it occupies the entire uterine cavity. During this stage of pregnancy, the uterus expands significantly to accommodate the growing fetus, which generally weighs between 5.5 to 8.8 pounds (or more) by the time of delivery. This expansion means that the uterus is not only filled but also adapted to support the size and needs of the fetus, including the necessary amniotic fluid.

Given this substantial growth and development, the correct answer reflects the anatomical and physiological changes that occur in the body during the final weeks of pregnancy. The uterus undergoes remarkable stretching and remodeling, which is essential for the successful delivery of the fetus. Thus, by the end of the gestational period, the entire uterus is filled by the fetus.